Radar systems are useful for detecting, characterizing and monitoring various kinematic parameters associated with natural and/or man-made objects and are critical to both civilian and military operations. These systems typically transmit “beams” or electromagnetic (EM) signals intended to engage one or more objects or targets, and process reflected return signals (or echoes) for object identification and characterization. A radar echo return usually contains both signals generated from a target, as well as background clutter. The clutter signal arises from reflections from stationary and slow-moving background objects (e.g. precipitation, terrain, etc.), and is usually stronger than the target signal. This clutter decreases radar performance by hindering the system's ability to detect targets and/or increases the probability of a false target detection.
Numerous methods exist which attempt to discriminate between unwanted clutter and target return signals. Many of these clutter cancellation methods rely on the principle that moving targets have a Doppler frequency shift, while stationary targets do not. Thus, pulse-Doppler radar systems may implement a plurality of Doppler frequency filters (e.g. FFT networks) used to divide the Doppler frequency space into many narrow regions, with each filter corresponding to one of these frequency bands. Knowing the frequency space normally associated with specific clutter types, these Doppler filters can be used to discriminate against clutter, as well as identify target Doppler frequency.
As radar sensitivity increases, previously undetectable wideband discrete clutter returns can cause high false alarm rates. In order to improve detectability, as well as improve the rejection of this clutter, fixed PRF, multi-pulse Doppler waveforms are typically used. As will be understood by one of ordinary skill in the art, due to the bandwidth of the clutter, fixed PRF Doppler waveforms with adequate Doppler visibility are often range ambiguous. Thus, in order to cover range and Doppler blinds inherent in this type of fixed PRF processing, as well as obtain detection that provides accurate range and angle information, transmitted beams must consist of multiple, unique high fixed PRF waveforms. Moreover, target hits must occur on more than one of these pulses in order to obtain accurate range data.
There are several additional problems with this waveform approach. First, the sensitivity of the system is impacted due to the coherent processing interval (CPI) being shortened to provide for multiple PRFs. The sensitivity is also impacted due to multiple hits being required to provide accurate range in the range ambiguous CPIs resulting from the high PRF. Further, due to small targets folding into short ranges and competing with large clutter, predetermined system clutter stability requirements are often difficult to achieve. Finally, processing ambiguous targets precludes the use of more sensitive Swerling (e.g. SW2/SW4) detection processing techniques.
Accordingly, improved methods of processing return signals in a pulse-Doppler radar system are desired.